Windows XP TCP/IP Corrupt?

I have two Windows XP Pro SP1 computers and a Sprint router hooked through a switch. Both computers access the internet fine. PC1 is currently sharing it's printer to the network. The first computer (PC1) works great. It sees itself and it's respective shares as well as the second computer (PC2) and it's respective shares under the defined workgroup. PC1 can access files and folders on PC2 with no problems.

However, PC2 does not work quite the same as PC1. I can see both PC1 and PC2 under my workgroup name in Network Places, but I can't view or access any shared resources defined on PC1. I get an error that says I don't have appropriate permissions. Confusingly, I am able to view and access the shared printer provided by PC1 under PC2's Printers folder. On PC2, I am also able to map a drive letter to a manually typed in folder share that I've created on PC1. However, browsing for the share there, gives me that permission error again when attempting to access PC1.

I had no problems with networking PC1 and PC2 directly together using just a crossover cable. This problem arised from installing a router, a switch, and new straight-through ethernet cables. Once this equipment was installed, the original problem I encountered was that PC1 and PC2 could see each other's computer name under the workgroup, but they couldn't display or access any shared resources between them. Pinging their respective IPs failed between them as well. Disabling all firewalls, re-running Network Setup Wizard multiple times on both PCs, adding IPX/SPX and enabling sharing exclusively with it as a workaround , and enabling NetBIOS over TCP/IP has gotten me to this point.

Why does Windows XP lose it's ability to share files and folders over a TCP/IP network once a switch and router is installed? How do I fix this so that I'm just using TCP/IP for my networking? Remember, everything originally worked just fine both directions with just a standard crossover cable.

Pondering this topic in my head, let me correct what I previously posted. I disabled all "software" firewalls installed on both PC1 and PC2. The router has a built-in firewall that is currently still enabled. Also, PC1 and PC2 had software firewalls running when they were working fine using a standard crossover cable. I'm wondering if the Windows XP firewall and the router's firewall may have conflicted with each other causing issues with TCP/IP?